Experimental Study On The Flow Structures Of Turbulent Boundary Layer Controlled With Micro-bubbles

Flow structures in the turbulent boundary layer can be modified by micro-bubbles,which can change the efficiency of the mass and heat transfer and the drag friction in the process of the fluid transport through changing the flow structures in the boundary layer.This approach is widely used in the fields of underwater ordnance,submarine,torpedo,et al.Therefore,it is of great theoretical and practical significance to study the turbulent boundary layer with the micro-bubbles.In this paper,the influence of turbulent boundary layer with micro-bubbles is studied by using the particle image velocimetry technology.The flow structures corresponding to different scales are analyzed.The main work is as the following:(1)The design and modification works of the low-speed circulating water tunnel are carried out.The flow stability devices are added on entrance and the downstream of the contraction section.The test section is redesigned and installed.The flexible connections are made between the water tunnel components.The contraction section,test section and the downstream diffusion section are accurately located at the same horizontal level.The flow stability devices reduces the vibration in the flow.The extended test section ensures the full development of the flow and the flexible connections avoid the influence of the vibrations among the components of the water tunnel.All of these modifications have effectively improved the quality of the water tunnel.(2)The influence on the mean velocity profile as well as the second order statistics of the fully developed turbulent boundary layer with the micro-bubbles is studied at different Reynolds numbers in the low-speed circulating water tunnel.The results show that the diameter of the micro-bubbles is in the range of 0.1-0.38mm at Re=0.87×10~5?1.23×10~5.The distribution of micro-bubbles is a single peak near the wall,i.e.,the micro-bubbles accumulates in the near wall region and decreases rapidly with the increase of the wall-normal distance.The micro-bubbles mainly have two effects on the flow structures,i.e.,buoyancy and extrusion effects.Part of the fluid is drived upward by the buoyancy effect,leading to an enhancement of the wall-normal turbulence intense of the boundary layer.At the same time,the density of the fluid is decreased and the flow structures in the turbulent boundary layer are pushed away from the near-wall region by the extrusion effect.The interactions between these two effects make the vorticity structures and turbulence activity region far away from the wall.The buoyancy effect is dominated when the Re is small,while the extrusion effect plays a more important role with the increase of Re.(3)The velocity fields of the single-phase fluid(fluid without micro-bubbles)and the mixed fluid(fluid with micro-bubbles)are measured in the streamwise and wall-normal plane.The effects of the micro-bubbles on the large scale structures in turbulent boundary layer are analyzed in views of coherent structures,energy content structures and topology structures.The results show that the micro-bubbles give rise to the decrease of the correlation of the coherent structures and the scales of the structures in streamwise and wall-normal are decreased.The micro-bubbles decrease the energy fractions of the energy content structures in the leading-order POD modes,and the effect decreases as the POD modes number increases.The topology analyses on the energy content structures in POD modes show that the deformation and rotation of the fluid in the mixed fluid are weaker than those of the single-phase fluid in the near wall region.The difference between the deformation and rotation for the mixed fluid becomes more obvious with the decrease of the scales of the energy content structures.(4)The velocity fields of the single-phase and the mixed fluids are measured in the streamwise-spanwise planes.The effects of the micro-bubbles on the high-and low-speed longitudinal streaky structures in the turbulent boundary layer are analyzed.The results based on the correlation function show that the micro-bubbles give rise to the decrease of the intensities of the streaky structures and the increase of the space between them.These changes decrease gradually with the increase of the wall-normal heights.The analysis of POD shows that the streaky structure activities are decreased while the non-streaky structures turbulent activities are enhanced for the mixed fluid.The enhancement is more obvious at a higher plane,which makes the effect of micro-bubbles on the energy content weaker with the increase of the wall-normal height.